Comparison of the time and latitude trends of melanoma incidence in anorectal region and perianal skin with those of cutaneous malignant melanoma in Norway

Background Melanoma incidence is increasing in many parts of the world. The main environmental risk factor is exposure to solar radiation. However, melanomas may arise also on non-sun-exposed areas (uveal and mucosal melanomas) and little is known about a possible relationship between sun exposure and melanoma on such locations. Objectives We have compared the time and latitude trends of melanoma incidence in the anorectal region and perianal skin (non-sun-exposed sites) with those of cutaneous malignant melanoma (CMM) (sun-exposed skin) to gain more information about the relationship between sun exposure and melanoma on such sites. Methods We analysed epidemiological data from the Cancer Registry of Norway for melanomas of the anorectal mucosa, perianal skin and overall CMM for the time period 1966-2007. Results We found that melanoma incidence on these shielded sites tends to decrease or remain constant over a period during which the CMM rates increase. This is true both in the North and in the South regions of Norway. Comparison of latitudinal trends of the incidence rates of CMM and melanoma on these shielded sites shows that there is no latitude gradient for melanoma of the anorectal mucosa and perianal skin, whereas there is a strong one for CMM. Conclusions The time and latitudinal trends are likely to support the assumption that melanomas on these shielded sites are not generated by ultraviolet radiation. Possible causes and significances of these trends are discussed.